EP 0 761 292 A1 discloses self-supporting capillary membranes for use at higher pressure with an internal separating layer. This known capillary membrane is provided with a seamless outside jacket made as a closed, liquid-permeable diagonal reinforcement. The diagonal reinforcement has individual threads including for their part either of monofilaments or multifilaments. The material selection for these monofilaments or multifilaments is almost unlimited. For example, polyester yarns, polyaramides, polymer fibers, glass fibers, or metal filaments are used. The individual threads of the diagonal reinforcement are applied to the capillaries in a 1:1 to 2:1 or even 2:2 braid binding in a continuous process as a seamless outer jacket. Depending on the wall thickness of the monofilaments or multifilaments, capillaries with outside diameters from 0.5 mm to 15 mm are produced. The capillaries themselves must have a mechanical stability that resists the thread tension in the reinforcing process. This known capillary membrane can be continuously produced. Due to the indicated diagonal reinforcement, it has a correspondingly high burst pressure; but the performance in filtration is limited. In spite of support by the liquid-permeable diagonal reinforcement, in severe filtration operation the membrane material surrounded in this way can fail.
DE 41 42 417 A1 discloses an ion exchanger membrane which, in order to eliminate these disadvantages, calls for reinforcement with a multiaxial, preferably triaxial, yarn system. In a chlor-alkali cell, using an open reinforcing fabric and a thin membrane to obtain low cell tension and good stability for handling of the reinforcing fabric and for the reinforced membrane is considered to be advantageous. A thin membrane in turn requires a thin fabric and low total thickness of the film layers used for lamination of the reinforced cation exchanger resin. By definition, an open fabric is one which, when there is illumination from one direction perpendicular to the plane of the fabric, enables a large portion of the incident light to pass through the fabric. This fabric with a high percentage of open points (fabric gaps) is desirable according to the known solution, because the open sites allow the cations to pass from the anolyte to the catholyte in a chlor-alkali process. An open fabric then enables a low cell tension, and therefore, lower power consumption. The known fabrics in the form of a basket weave or a cross weave are regarded as disadvantageous because they lack dimensional stability and have a low modulus and low strength in the oblique direction of the fabric laminate. Accordingly, the known solution proposes a triaxially woven article, that is, a fabric produced by a weaving process, in which three yarns at an angle of 60° each to one another are used. In particular, if the triaxial fabric is produced from fine yarns, a thin fabric can be obtained that is stable under various loads, even if the fabric has the desired high degree of open sites. The disadvantage in this known solution is that the indicated multiaxial fabric is relatively complex and expensive to produce, making the membrane material more expensive. Furthermore, this solution is limited in filtration performance by its basic structure of the membrane material.
EP 0 355 400 A2 discloses a filter medium for caking filtration processes with a filter membrane embedded on either side in a fabric construction assuming a mechanical protective function. With this known filter medium, within filter systems in which the filter medium is mechanically loaded to an extremely heavy degree, suspensions can be processed and particles in the submicroporous range filtered out of the fluid flow. In one preferred embodiment of the known solution the membrane mass can be located between two fabric plies. The membrane mass is applied to a compact, strong carrier fabric ply. A protective fabric ply is embedded into the upper layer of the membrane mass. This protective fabric ply is made as an open-mesh fabric, achieving a sandwich structure for the filter medium for caking filtration processes. This generic filter-medium has risen to the robust mechanical requirements conventionally imposed by filtration technology. Especially when the filter cake is removed or discarded, the filter medium is exposed to considerable mechanical loads which can be managed with the known filter medium. Still, the filtration performance is not satisfactory even in this durable solution.